Skeletal adaptation to mechanical load causes patients subject to temporary paralysis, debilitation, or bedrest to rapidly lose significant bone mass, density, and strength leading to osteopenia/osteoporosis and an increased risk of fragility fractures. Therapies aimed at preventing disuse-induced bone loss or enhancing the anabolic response to increased load would reduce fracture rates. However, the cellular and molecular mechanisms regulating these processes are poorly understood. The current proposal will investigate the contribution of Sirt1 in disuse-induced bone loss and the skeletal adaptation to increased loading. The sirt1 gene product, Sirtuin 1, is a histone deacetylase (HDAC) that has been termed an anti-aging gene. However, Sirt1 also controls the activity of many other proteins including - catenin, a mediator of skeletal adaptation to load. Preliminary data demonstrate that Sirtuin 1 protein is expressed in osteoblasts, osteocytes, and osteoclasts. Conditional deletion of sirt1 in both osteoblasts and osteocytes in mice leads to a low bone mass/volume phenotype by 4 months of age. A uniaxial tibial loading study surprisingly revealed that the long bones of Sirt1Ob-/- mice were hypersensitive to the anabolic effect of increased loading. Additional data revealed that deletion of Sirt1 suppresses Wnt signaling, which inhibits the bone forming activity of mature osteoblasts. Hence the hypothesis for this proposal is that Sirt1 normally suppresses the bone response to increased loading and promotes disuse osteopenia by permitting activation of canonical Wnt signaling in mature osteoblasts. This hypothesis will be explored in the following three aims. Aim 1 - Determine whether deletion of Sirt1 in osteoblasts and/or osteocytes enhances bone formation and Wnt signaling in response to increased mechanical loading. Aim 2 - Determine whether deletion of Sirt1 in osteoblasts and/or osteocytes accelerates disuse osteopenia and enhances the recovery of bone following disuse. Aim 3 - Determine the effects of Sirt1 on Wnt signaling and -catenin transcriptional activity in osteoblasts and osteocytes and identify additional Sirt1 substrates in osteoblasts. These studies will determine whether Sirt1 and its substrates in osteoblasts and/or osteocytes are potential therapeutic targets for novel interventions for disuse osteopenia and, potentially, bone loss from a variety of other causes.